[PATCH v3 0/4] rust: a few common Borrow/BorrowMut implementations

[PATCH v3 0/4] rust: a few common Borrow/BorrowMut implementations

[Alexandre Courbot]

The Borrow trait has multiple uses, one of them being to store either an
owned value or a reference to it inside a generic container. This series
adds these implementations for `Box`, `Arc`, `Vec`, and `CString`. I
came across the need for this while experimenting with the scatterlist
abstraction series [1].

This second revision adds examples to each impl block as requested. I am
personally not quite convinced that they are needed, as they just
illustrate the basic usage of `Borrow`, but have added them to this
revision so we can decide whether we want them or not.

[1] https://lore.kernel.org/rust-for-linux/DA9JTYA0EQU8.26M0ZX80FOBWY@nvidia.com/

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
Changes in v3:
- Remove undeeded first line of doccomments.
- Link to v2: https://lore.kernel.org/r/20250613-borrow_impls-v2-0-6120e1958199@nvidia.com

Changes in v2:
- Rebase on top of v6.16-rc1.
- Improve commit messages. (thanks Benno!)
- Add examples on each impl block.
- Link to v1: https://lore.kernel.org/r/20250601-borrow_impls-v1-0-e1caeb428db4@nvidia.com

---
Alexandre Courbot (4):
      rust: alloc: implement `Borrow` and `BorrowMut` for `Vec`
      rust: sync: implement `Borrow` and `BorrowMut` for `Arc` types
      rust: alloc: implement `Borrow` and `BorrowMut` for `KBox`
      rust: str: implement `Borrow` and `BorrowMut` for `CString`

 rust/kernel/alloc/kbox.rs | 57 ++++++++++++++++++++++++++++++++++
 rust/kernel/alloc/kvec.rs | 53 ++++++++++++++++++++++++++++++++
 rust/kernel/str.rs        | 45 +++++++++++++++++++++++++++
 rust/kernel/sync/arc.rs   | 78 ++++++++++++++++++++++++++++++++++++++++++++++-
 4 files changed, 232 insertions(+), 1 deletion(-)
---
base-commit: 19272b37aa4f83ca52bdf9c16d5d81bdd1354494
change-id: 20250531-borrow_impls-8dfef3fcee93

Best regards,
-- 
Alexandre Courbot <acourbot@nvidia.com>

[PATCH v3 1/4] rust: alloc: implement `Borrow` and `BorrowMut` for `Vec`

[Alexandre Courbot]

Implement `Borrow<[T]>` and `BorrowMut<[T]>` for `Vec<T>`. This allows
`Vec<T>` to be used in generic APIs asking for types implementing those
traits. `[T; N]` and `&mut [T]` also implement those traits allowing
users to use either owned, borrowed and heap-owned values.

The implementation leverages `as_slice` and `as_mut_slice`.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/alloc/kvec.rs | 53 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index 1a0dd852a468ccda6ea1b521bc1e7dbc8d7fc79c..cc8e7499427181e79af4a54576a7af23ac8b1bb9 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -8,6 +8,7 @@
     AllocError, Allocator, Box, Flags,
 };
 use core::{
+    borrow::{Borrow, BorrowMut},
     fmt,
     marker::PhantomData,
     mem::{ManuallyDrop, MaybeUninit},
@@ -890,6 +891,58 @@ fn deref_mut(&mut self) -> &mut [T] {
     }
 }
 
+/// # Examples
+///
+/// ```
+/// # use core::borrow::Borrow;
+/// struct Foo<B: Borrow<[u32]>>(B);
+///
+/// // Owned array.
+/// let foo_array = Foo([1, 2, 3]);
+///
+/// // Owned vector.
+/// let foo_vec = Foo(KVec::from_elem(0, 3, GFP_KERNEL)?);
+///
+/// let arr = [1, 2, 3];
+/// // Borrowed slice from `arr`.
+/// let foo_borrowed = Foo(&arr[..]);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T, A> Borrow<[T]> for Vec<T, A>
+where
+    A: Allocator,
+{
+    fn borrow(&self) -> &[T] {
+        self.as_slice()
+    }
+}
+
+/// # Examples
+///
+/// ```
+/// # use core::borrow::BorrowMut;
+/// struct Foo<B: BorrowMut<[u32]>>(B);
+///
+/// // Owned array.
+/// let foo_array = Foo([1, 2, 3]);
+///
+/// // Owned vector.
+/// let foo_vec = Foo(KVec::from_elem(0, 3, GFP_KERNEL)?);
+///
+/// let mut arr = [1, 2, 3];
+/// // Borrowed slice from `arr`.
+/// let foo_borrowed = Foo(&mut arr[..]);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T, A> BorrowMut<[T]> for Vec<T, A>
+where
+    A: Allocator,
+{
+    fn borrow_mut(&mut self) -> &mut [T] {
+        self.as_mut_slice()
+    }
+}
+
 impl<T: Eq, A> Eq for Vec<T, A> where A: Allocator {}
 
 impl<T, I: SliceIndex<[T]>, A> Index<I> for Vec<T, A>

-- 
2.49.0

[PATCH v3 2/4] rust: sync: implement `Borrow` and `BorrowMut` for `Arc` types

[Alexandre Courbot]

Implement `Borrow<T>` and `BorrowMut<T>` for `UniqueArc<T>`, and
`Borrow<T>` for `Arc<T>`. This allows these containers to be used in
generic APIs asking for types implementing those traits. `T` and `&mut
T` also implement those traits allowing users to use either owned,
shared or borrowed values.

`ForeignOwnable` makes a call to its own `borrow` method which must be
disambiguated.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/sync/arc.rs | 78 ++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 77 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index c7af0aa48a0a049bfeeba3a81080355f4d381738..31d6f5946d554602ff689a452df7f2b229eeef71 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -25,6 +25,7 @@
 };
 use core::{
     alloc::Layout,
+    borrow::{Borrow, BorrowMut},
     fmt,
     marker::PhantomData,
     mem::{ManuallyDrop, MaybeUninit},
@@ -406,7 +407,7 @@ unsafe fn borrow<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
     unsafe fn borrow_mut<'a>(ptr: *mut Self::PointedTo) -> ArcBorrow<'a, T> {
         // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
         // requirements for `borrow`.
-        unsafe { Self::borrow(ptr) }
+        unsafe { <Self as ForeignOwnable>::borrow(ptr) }
     }
 }
 
@@ -426,6 +427,31 @@ fn as_ref(&self) -> &T {
     }
 }
 
+/// # Examples
+///
+/// ```
+/// # use core::borrow::Borrow;
+/// # use kernel::sync::Arc;
+/// struct Foo<B: Borrow<u32>>(B);
+///
+/// // Owned instance.
+/// let foo_owned = Foo(1);
+///
+/// // Shared instance.
+/// let arc = Arc::new(1, GFP_KERNEL)?;
+/// let foo_shared = Foo(arc.clone());
+///
+/// let i = 1;
+/// // Borrowed from `i`.
+/// let foo_borrowed = Foo(&i);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T: ?Sized> Borrow<T> for Arc<T> {
+    fn borrow(&self) -> &T {
+        self.deref()
+    }
+}
+
 impl<T: ?Sized> Clone for Arc<T> {
     fn clone(&self) -> Self {
         // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
@@ -834,6 +860,56 @@ fn deref_mut(&mut self) -> &mut Self::Target {
     }
 }
 
+/// # Examples
+///
+/// ```
+/// # use core::borrow::Borrow;
+/// # use kernel::sync::UniqueArc;
+/// struct Foo<B: Borrow<u32>>(B);
+///
+/// // Owned instance.
+/// let foo_owned = Foo(1);
+///
+/// // Owned instance using `UniqueArc`.
+/// let arc = UniqueArc::new(1, GFP_KERNEL)?;
+/// let foo_shared = Foo(arc);
+///
+/// let i = 1;
+/// // Borrowed from `i`.
+/// let foo_borrowed = Foo(&i);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T: ?Sized> Borrow<T> for UniqueArc<T> {
+    fn borrow(&self) -> &T {
+        self.deref()
+    }
+}
+
+/// # Examples
+///
+/// ```
+/// # use core::borrow::BorrowMut;
+/// # use kernel::sync::UniqueArc;
+/// struct Foo<B: BorrowMut<u32>>(B);
+///
+/// // Owned instance.
+/// let foo_owned = Foo(1);
+///
+/// // Owned instance using `UniqueArc`.
+/// let arc = UniqueArc::new(1, GFP_KERNEL)?;
+/// let foo_shared = Foo(arc);
+///
+/// let mut i = 1;
+/// // Borrowed from `i`.
+/// let foo_borrowed = Foo(&mut i);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T: ?Sized> BorrowMut<T> for UniqueArc<T> {
+    fn borrow_mut(&mut self) -> &mut T {
+        self.deref_mut()
+    }
+}
+
 impl<T: fmt::Display + ?Sized> fmt::Display for UniqueArc<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt::Display::fmt(self.deref(), f)

-- 
2.49.0

[PATCH v3 3/4] rust: alloc: implement `Borrow` and `BorrowMut` for `KBox`

[Alexandre Courbot]

Implement `Borrow<T>` and `BorrowMut<T>` for `KBox<T>`. This allows
`KBox<T>` to be used in generic APIs asking for types implementing those
traits. `T` and `&mut T` also implement those traits allowing users to
use either owned, borrowed and heap-owned values.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/alloc/kbox.rs | 57 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index c386ff771d506a2eb4c211a93ea9b59bc04c93f5..e60e8c9b4d8539db6b8861ce9b8ae610ada4ddcb 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -6,6 +6,7 @@
 use super::allocator::{KVmalloc, Kmalloc, Vmalloc};
 use super::{AllocError, Allocator, Flags};
 use core::alloc::Layout;
+use core::borrow::{Borrow, BorrowMut};
 use core::fmt;
 use core::marker::PhantomData;
 use core::mem::ManuallyDrop;
@@ -499,6 +500,62 @@ fn deref_mut(&mut self) -> &mut T {
     }
 }
 
+/// # Examples
+///
+/// ```
+/// # use core::borrow::Borrow;
+/// # use kernel::alloc::KBox;
+/// struct Foo<B: Borrow<u32>>(B);
+///
+/// // Owned instance.
+/// let foo_owned = Foo(1);
+///
+/// // Owned instance using `KBox`.
+/// let foo_box = Foo(KBox::new(1, GFP_KERNEL)?);
+///
+/// let i = 1;
+/// // Borrowed from `i`.
+/// let foo_borrowed = Foo(&i);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T, A> Borrow<T> for Box<T, A>
+where
+    T: ?Sized,
+    A: Allocator,
+{
+    fn borrow(&self) -> &T {
+        self.deref()
+    }
+}
+
+/// # Examples
+///
+/// ```
+/// # use core::borrow::BorrowMut;
+/// # use kernel::alloc::KBox;
+/// struct Foo<B: BorrowMut<u32>>(B);
+///
+/// // Owned instance.
+/// let foo_owned = Foo(1);
+///
+/// // Owned instance using `KBox`.
+/// let foo_box = Foo(KBox::new(1, GFP_KERNEL)?);
+///
+/// let mut i = 1;
+/// // Borrowed from `i`.
+/// let foo_borrowed = Foo(&mut i);
+/// # Ok::<(), Error>(())
+/// ```
+impl<T, A> BorrowMut<T> for Box<T, A>
+where
+    T: ?Sized,
+    A: Allocator,
+{
+    fn borrow_mut(&mut self) -> &mut T {
+        self.deref_mut()
+    }
+}
+
 impl<T, A> fmt::Display for Box<T, A>
 where
     T: ?Sized + fmt::Display,

-- 
2.49.0

[PATCH v3 4/4] rust: str: implement `Borrow` and `BorrowMut` for `CString`

[Alexandre Courbot]

Implement `Borrow<CStr>` and `BorrowMut<CStr>` for `CString`. This
allows `CString` to be used in generic APIs asking for types
implementing those traits. `&CStr` and `&mut CStr` also implement those
traits allowing users to use either owned or borrowed values.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/str.rs | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index a927db8e079c3597860880947a03959e1d6d712e..1cf787d4ace36a84d0ece7a7017ef0c67499bf89 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs
@@ -3,6 +3,7 @@
 //! String representations.
 
 use crate::alloc::{flags::*, AllocError, KVec};
+use core::borrow::{Borrow, BorrowMut};
 use core::fmt::{self, Write};
 use core::ops::{self, Deref, DerefMut, Index};
 
@@ -911,6 +912,50 @@ fn deref_mut(&mut self) -> &mut Self::Target {
     }
 }
 
+/// # Examples
+///
+/// ```
+/// # use core::borrow::Borrow;
+/// # use kernel::str::{CStr, CString};
+/// # use kernel::fmt;
+/// struct Foo<B: Borrow<CStr>>(B);
+///
+/// // Owned instance using `CString`.
+/// let foo_owned = Foo(CString::try_from_fmt(fmt!("{}", "abc"))?);
+///
+/// let str_data = b"abc\0";
+/// // Borrowed from `str_data`.
+/// let foo_borrowed = Foo(CStr::from_bytes_with_nul(str_data)?);
+/// # Ok::<(), Error>(())
+/// ```
+impl Borrow<CStr> for CString {
+    fn borrow(&self) -> &CStr {
+        self.deref()
+    }
+}
+
+/// # Examples
+///
+/// ```
+/// # use core::borrow::BorrowMut;
+/// # use kernel::str::{CStr, CString};
+/// # use kernel::fmt;
+/// struct Foo<B: BorrowMut<CStr>>(B);
+///
+/// // Owned instance using `CString`.
+/// let foo_owned = Foo(CString::try_from_fmt(fmt!("{}", "abc"))?);
+///
+/// let mut str_data = [b'a', b'b', b'c', 0];
+/// // Borrowed from `str_data`.
+/// let foo_borrowed = Foo(unsafe { CStr::from_bytes_with_nul_unchecked_mut(&mut str_data) });
+/// # Ok::<(), Error>(())
+/// ```
+impl BorrowMut<CStr> for CString {
+    fn borrow_mut(&mut self) -> &mut CStr {
+        self.deref_mut()
+    }
+}
+
 impl<'a> TryFrom<&'a CStr> for CString {
     type Error = AllocError;
 

-- 
2.49.0

Re: [PATCH v3 4/4] rust: str: implement `Borrow` and `BorrowMut` for `CString`

[Miguel Ojeda]

On Sun, Jun 15, 2025 at 2:37 PM Alexandre Courbot <acourbot@nvidia.com> wrote:
>
> +/// // Borrowed from `str_data`.
> +/// let foo_borrowed = Foo(unsafe { CStr::from_bytes_with_nul_unchecked_mut(&mut str_data) });

We will need a `// SAFETY:` comment -- Clippy should complain.

Or to add a safe `from_bytes_with_nul_mut`, I guess.

Cheers,
Miguel

Re: [PATCH v3 4/4] rust: str: implement `Borrow` and `BorrowMut` for `CString`

[Alexandre Courbot]

On Sun Jun 15, 2025 at 10:15 PM JST, Miguel Ojeda wrote:
> On Sun, Jun 15, 2025 at 2:37 PM Alexandre Courbot <acourbot@nvidia.com> wrote:
>>
>> +/// // Borrowed from `str_data`.
>> +/// let foo_borrowed = Foo(unsafe { CStr::from_bytes_with_nul_unchecked_mut(&mut str_data) });
>
> We will need a `// SAFETY:` comment -- Clippy should complain.

I thought I could get away with it since it is in a test, and clippy
does not seem to complain about it, but...

>
> Or to add a safe `from_bytes_with_nul_mut`, I guess.

... as penance (and for symmetry with `from_bytes_with_nul`), let me
implement that so we don't need a safety block at all. :)

Re: [PATCH v3 4/4] rust: str: implement `Borrow` and `BorrowMut` for `CString`

[Miguel Ojeda]

On Sun, Jun 15, 2025 at 3:48 PM Alexandre Courbot <acourbot@nvidia.com> wrote:
>
> I thought I could get away with it since it is in a test, and clippy
> does not seem to complain about it, but...

Hmm... It should:

error: unsafe block missing a safety comment
    --> rust/doctests_kernel_generated.rs:7973:24
     |
7973 | let foo_borrowed = Foo(unsafe {
CStr::from_bytes_with_nul_unchecked_mut(&mut str_data) });
     |
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Do you have the doctests enabled? i.e. CONFIG_RUST_KERNEL_DOCTESTS=y

> ... as penance (and for symmetry with `from_bytes_with_nul`), let me
> implement that so we don't need a safety block at all. :)

:)

Cheers,
Miguel

Re: [PATCH v3 4/4] rust: str: implement `Borrow` and `BorrowMut` for `CString`

[Alexandre Courbot]

On Sun Jun 15, 2025 at 10:57 PM JST, Miguel Ojeda wrote:
> On Sun, Jun 15, 2025 at 3:48 PM Alexandre Courbot <acourbot@nvidia.com> wrote:
>>
>> I thought I could get away with it since it is in a test, and clippy
>> does not seem to complain about it, but...
>
> Hmm... It should:
>
> error: unsafe block missing a safety comment
>     --> rust/doctests_kernel_generated.rs:7973:24
>      |
> 7973 | let foo_borrowed = Foo(unsafe {
> CStr::from_bytes_with_nul_unchecked_mut(&mut str_data) });
>      |
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> Do you have the doctests enabled? i.e. CONFIG_RUST_KERNEL_DOCTESTS=y

... and that's what I was missing. ^_^; Thanks!